A variety of approaches have been used for x-ray imaging. X-ray film is perhaps the most basic approach. X-ray film provides reasonable resolution, and has a compact form factor, but does not provide real time imaging. The film must be exposed and then developed before the image can be viewed. The developing process uses environmentally hazardous chemicals, and the exposure, develop, analysis cycle must sometimes be repeated several times before the desired image is created. In addition, the detection efficiency of x-ray film is less than ideal for many applications.
X-ray image intensifiers can be combined with television cameras to provide real time imaging, but they are bulky and have limited resolution.
Computed radiography has a small form factor, and electronic readout, but the resolution and detection efficiency are low and computed radiography does not provide fast readout.
There is a need for flat panel x-ray detectors, both direct and indirect sensing types, and a variety of such detectors are presently in development which overcome many of the limitations just mentioned, but have not achieved acceptable electronic noise and resolution performance.
One approach presently being developed uses an electron beam to read out an image stored on an x-ray sensitive photo-conductive target. Devices of this type are described in U.S. Pat. No. 5,195,118. The target is first charged to a uniform negative potential, for example by scanning it with the electron beam. Incident x-rays cause localized discharge to form a latent image on the target. As long as the target resistivity is high enough, the charge pattern representing the image will remain spatially localized.
The image is read by scanning the target with the electron beam in raster fashion. This serves the purpose of both recharging the target to its initial potential and creating a current signal proportional to the latent charge image. The current flowing in the electron beam is then sensed by an output amplifier. As the electron beam is scanned across the target, the amplifier produces a video signal representing the latent image on the target. Target materials can be produced that have very high spatial resolution. The overall resolution of the detector is limited by the size and shape of the electron beam.
An alternative approach also utilizing a photoconductive target uses an array of cold cathode field emitters of the type used in field emitter displays to supply an addressable source of electrons. Detectors, using such emitters, are described in U.S. Pat. No. 5,567,929. The resolution achievable by this approach is limited by the shape of the electron beam created as electrons leave the hemispherical tip of the elements of the cathode. In field emitter based displays, the beam can be narrowed by utilizing a high voltage anode, or by placing the display phosphor (in a field emitter display) close to the cathode. These approaches are not applicable to imagers using photoconductive detectors, because the landing velocity of the electrons must be small, thus preventing a high voltage from being used, and the target layer must be spaced farther away from the emitter layer to reduce output noise. Output noise is proportional to the capacitance, and the capacitance is inversely proportional to the distance between the target electrode and all other physical structures in the imager. In order to create a high speed scanned system, a large beam current is required in order to recharge each picture element of the target during the time the beam impinges on that pixel. Field emitter arrays typically have current limiting resistors and/or exhibit large variations in current from tip to tip, due to process non-uniformities. This limits the beam current, and therefore limits the readout speed achievable with field emitters. Other problems make this approach difficult, including the need for the addressable array of field emitters to be inside a vacuum envelope. The addressing circuitry that drives each row and senses each column must be outside the envelope, and this creates the need for many electrical feed throughs into the vacuum envelope, introducing manufacturing difficulties. Moreover, the device cannot be completely tested until it is assembled in the vacuum envelope.
There is a need for an x-ray imager that overcomes the disadvantages of the prior art. More specifically, there is a need for an imager that has a small form factor, that is, an imager that is approximately as thin and flat as an x-ray film cassette, has electronic readout as opposed to film which must be scanned to provide digital images, has a wide dynamic range (1000:1) and has a high detection efficiency (50%). For fluoroscopy, the imager has low electronic noise, below the quantum noise of the x-ray image and fast readout (at least 30 frames per second). For radiographic imaging the imager has high resolution (&gt;5 lp/mm).
It is an object of this invention to provide a high resolution flat panel imager for x-ray or other radiation sources that overcomes the disadvantages of the imagers just discussed, and provides the characteristics just mentioned.